1. Field of the Invention
The present invention relates to a light amount adjustment device for adjusting the amount of light passing therethrough and relates to an optical apparatus having the light amount adjustment device.
2. Description of the Related Art
Conventionally, an image pickup apparatus such as a digital camera or a digital single-lens reflex camera (hereinafter referred to as the single-lens reflex camera) is mounted with a light amount adjustment device such as an aperture device (diaphragm device) or a shutter device.
In recent years, a single-lens reflex camera capable of still image photographing and moving image photographing is commercially available. This type of single-lens reflex camera has an interchangeable lens mounted with an aperture device as a light amount adjustment device. The aperture device must be capable of operating at high speed at still image photographing to increase the continuous photographing speed. At moving image photographing, the aperture device must be capable of operating with high resolution since an aperture reducing operation performed at low resolution entails an unnatural change in the amount of light, which lowers the quality of moving images.
Conventionally, the aperture device mounted to an interchangeable lens of a single-lens reflex camera is configured to be driven by a single motor. The motor is driven, within a characteristic range of the motor, at high speed for high-speed operation of the aperture device and driven at low speed for high-resolution operation of the aperture device. In the case, for example, of the aperture device having a stepping motor as drive source, the motor is driven at high speed by normal step driving such as 1-2 phase excitation driving and is smoothly driven at low speed by micro-step driving.
Japanese Laid-open Patent Publication No. 62-240942 discloses an electromagnetically driven aperture device that has a single stepping motor as drive source. In this device, the stepping motor is driven by 1-2 phase excitation driving at continuous still image photographing to ensure motor's high-speed driving performance at still image photographing, and is driven by micro-step driving at moving image photographing to ensure motor's low-speed driving performance at moving image photographing.
However, since the aperture device disclosed in Japanese Laid-open Patent Publication No. 62-240942 is configured to be driven by the single stepping motor, both the high-speed driving performance and the low-speed driving performance vary depending on the performance of the single stepping motor. Thus, the design of the aperture device is forced to balance the high- and low-speed driving performances.
As a result, in the case for example of increasing the magnetic flux of a rotor magnet so as to satisfy the requested high-speed driving performance, the smoothness of the motor at low-speed driving is impaired by the increase of cogging torque. It is therefore difficult to realize both the high-speed operation and the high-resolution operation of the aperture device, which poses a problem.